Portable communication devices, such as cellular telephones, personal digital assistants (PDAs), WIFI transceivers, and other communication devices transmit and receive communication signals at various frequencies that correspond to different communication bands and at varying power levels. A power amplifier module, generally comprising one or more amplification stages, is used to transmit the communication signals. A radio frequency (RF) power amplifier system may include multiple amplification stages, and, in some applications, multiple amplification paths.
A quadrature power amplifier is one in which the power amplifier is divided into two “paths,” a so-called “in-phase” (I) path and a so-called “quadrature” (Q) path. The in-phase path and the quadrature path are ideally separated in phase by 90 degrees. In one architecture, forward power detection in a quadrature power amplifier is performed by using a dual detector arrangement to sample the power in each of the in-phase and quadrature paths. The sampled power for each path is added together to obtain the total power. However, a dual detector arrangement exhibits accurate directionality at only 90 degree and 270 degree load phase angle, resulting in detector error at load phase angles other than 90 and 270 degrees. Further, this error worsens as voltage standing wave ratio (VSWR) increases.
FIG. 1 is a graphical diagram 10 illustrating an example of the output of a quadrature power amplifier with dual detectors. The dual detector architecture uses independent detectors on the in-phase (I) and quadrature (Q) paths whose average, or sum, represents the total power. In an implementation that uses bipolar junction transistor (BJT) technology to implement the amplification devices, the vertical axis 12 represents the output stage collector voltage of the in-phase path and the horizontal axis 14 represents the output stage collector voltage of the quadrature path. With a nominal output load, the in-phase path and quadrature paths are separated in phase by 90°.
The point 16 represents the voltage (i.e., the amplitude of the signal at the collector of the output stage) provided by the in-phase path and the point 18 represents the voltage (i.e., the amplitude of the signal at the collector of the output stage) provided by the quadrature path when the power amplifier has a nominal output load. However, in a real amplifier system having a VSWR at the output of the power amplifier, the point 16 rarely coincides exactly with the axis 12, and instead, is better represented as a circle 22 in which the point representing the amplitude of the signal at the collector of the output of the in-phase path can fall anywhere on the circumference of the circle 22 depending on the phase of the load. The radius of the circle 22 is proportional to the VSWR, with a higher VSWR corresponding to a larger circle. Similarly, the point 18 rarely coincides with the axis 14, and is better represented by a circle 24 in which the point representing the amplitude of the signal at the collector of the output of the quadrature path will fall on the circumference of the circle 24 at a location that is 180° opposite phase to the point representing the amplitude of the signal of the in-phase path. Using the quadrature path as an example, and equally applicable to the in-phase path, an arbitrary load phase at the output of the power amplifier is represented by vector 34 having an origin at point 18 and an angle 36 relative to a line 40 that is perpendicular to the axis 14 of the quadrature path. The magnitude of the vector 34 is determined by the VSWR at the output of the power amplifier.
The varying load phase angle 36 causes the point of the vector 28, which represents the detected RF voltage output of the quadrature path, to appear at a locus of locations on the circle 24 defined by the load phase angle 36. Similarly, a varying load phase angle will cause the vector 26 which represents the detected RF voltage output of the in-phase path, to appear at a locus of locations on the circle 22 defined by a load phase angle associated with a vector 42 having an origin at the point 16, and a load phase angle 44 relative to a line 48 that is perpendicular to the axis 12 of the in-phase path. Similar to the vector 34, the magnitude of the vector 42 is determined by the VSWR at the output of the power amplifier. The vector 42, relative to the line 48 that is perpendicular to the in-phase axis 12, is 180 out of phase from the vector 34, relative to the line 38 that is perpendicular to the quadrature axis 14. The load phase angle 36 is equal to the load phase angle 44, but the vectors 34 and 42 are 180 opposite in phase relative to the perpendicular lines 40 and 48, respectively. The vector 26 represents the detected RF voltage output of the in-phase path. The position of vector 26 on circle 22 is 180° out of phase from the position of vector 28 on circle 24. The above phase relationship is governed by the behavior of an ideal quadrature output matching network with load VSWR>1.
The time phase angle 32 represents the time phase of the vector 26 associated with the in-phase path and the time phase of the vector 28 (angle not shown) associated with the quadrature path relative to time. Accordingly, using the in-phase vector 26 as an example, but equally applicable to the quadrature vector 28, a peak error 38 in the detected RF voltage will exist because of the varying magnitude of the vector 26, which is determined by its location on the circle 22. Similarly, the varying magnitude of the vector 28 is determined by its location on the circle 24. This peak error 38 is a function of the load phase angle 44 for the in phase path and the load phase angle 36 for the quadrature path. Essentially this means that only at a load phase angle of 90° and at 270°, where the vectors 34 and 42 align with the axes 14 and 12, respectively, will the sum of the in-phase amplitude, represented by vector 26, and the quadrature amplitude, represented by vector 28 be an accurate representation of the power output.
FIG. 2 is a graphical illustration 50 showing power detector output voltage as a function of the load phase angle for a quadrature power amplifier with dual detectors. The dual detectors are independent detectors on the I and Q paths whose average or sum represents the total power. The horizontal axis 52 represents load phase angle and the vertical axis 54 represents detector output voltage. The traces 62, 64 and 66 represent the sum of the amplitude of the in-phase vector 26 and the quadrature vector 28 at VSWR 2:1; VSWR 4:1 and VSWR 6:1, respectively. As illustrated, it is clear that the detector output exhibits error at all load phases other than 90° and 270°; and furthermore, that the detector output error increases with increasing VSWR.
Thus, a dual detector arrangement will tend to indicate a power that is higher than the actual forward power resulting in an artificially lowered output power into a load having a VSWR greater than one, when used in a closed loop power control system.